This invention relates to the production of sodium aluminosilicate seed solutions intended to be used to initiate the crystallization of zeolitic sodium aluminosilicate molecular sieves from sources of sodium oxide, aluminum oxide and silicon dioxide. More particularly, the invention relates to improvements in the production of a clear seed solution to be used, for example, to initiate crystallization of zeolitic molecular sieves by the so-called "external" zeolite growth initiator (seed) procedure described in U.S. Pat. No. 4,493,902, "Fluid Catalytic Cracking Catalyst Comprising Microspheres Containing More Than About 40 Percent by Weight Y-Faujasite and Methods of Making the Same." The teachings of U.S. Pat. No. 4,493,902, including test procedures cross-referenced therein, are incorporated herein by reference thereto.
The process of the '902 patent generally involves immersing preformed porous microspheres comprising a mixture of calcined kaolin clays, in an alkaline sodium silicate reaction solution to crystallize high levels of sodium zeolite Y, above 40% as determined by X-ray diffraction, throughout the microspheres. This is followed by removal of mother liquor from the crystallized microspheres which are then ion-exchanged with suitable cations or combinations of cations to reduce the sodium oxide level to low levels. The resulting fluid cracking catalyst microspheres are characterized by exceptionally high activity, desirable selectivity and, in most preferred embodiments, an attrition-resistance better than or comparable to currently available fluid cracking catalysts.
In one embodiment of the invention, the reaction is "internally" seeded. That is, the seeding material is contained in precursor microspheres containing calcined kaolin clay. In another embodiment of the invention, the reaction is "externally" seeded, i.e., the growth initiator is added to the aqueous slurry containing precursor microspheres and sodium silicate solution. The '902 patent discloses (col. 13, 1. 50 to 56) that the mixture containing amorphous zeolite initiator (seeds) is preferably clear when external seeding is employed. The patent states that external seeds that are cloudy can result in crystallization products having a higher EAI (poorer attrition-resistance) than is desired and that excessive fines can be generated during the crystallization process when cloudy seeds are used.
To produce the desired clear seed solution the '902 patent discloses that sodium silicate, sodium aluminate and sodium hydroxide solutions are mixed to produce a solution of desired Na.sub.2 O, Al.sub.2 O.sub.3, SiO.sub.2 and H.sub.2 O content. The ingredients used to form the clear seed solution are heated at 100.degree. F. before mixing and are mixed in a controlled manner. The resulting mixture is covered and aged quiescently for about 6 hours at 100.degree. F. (to mature the seeds) and the clear solution is then added to a reactor along with a mixture of sodium silicate solution and microspheres of calcined clay. The '902 patent indicates (col. 13, 1. 40-45) that the mixture is clear when solutions of sodium silicate and sodium aluminate are mixed but that after aging at about 100.degree. F. for about 24 hours or longer, the solution begins to acquire a cloudy appearance.
It is now believed that cloudy seeds contain larger particles which cannot penetrate into the pores of the microspheres of calcined clay, whereby zeolite tends to grow on the surface of the microspheres instead of within the microspheres. Consequently, crystallized microspheres may be more porous and less attrition resistant. It is not presently known why the use of cloudy seeds may contribute to the generation of excessive fines in the reactor.
Prior to the present invention, it was recognized that it would be desirable to extend the period over which a clear seed solution could be stored without becoming cloudy. It was discovered that the useful life of the seed solution could be extended by cooling the solution. For example, in some cases, the mature seed solution would remain clear for as long as about 48 hours if the solution were cooled appreciably below 100.degree. F., e.g., 70.degree. F., by external cooling means. By extending the useful life of the seed solution a single large batch could be fed to a multiplicity of reactors. If the solution were not cooled, the time available for introducing the cooled seed solution with the reactants would be short. This would have a deleterious effect on the efficient operation of the process in a commercial plant since it would make it difficult to use a single batch of seed solution to feed a plurality of reactors. However, cooling is an expensive means to prolong the useful life of a clear seed solution, especially so when the seed solution is to be stored for 48 hours or longer.